When an X ray or a gamma ray is irradiated from the outside of a body, the dose becomes maximum in the vicinity of the surface of the body, and then the dose decreases as the depth increased. Accordingly, when it is tried to apply a sufficient dose to a tumor volume located in a deep position, there is caused larger damage to normal cells located at a shallower position than the tumor volume. In contrast, in the case of particle-beam irradiation, irradiation energy determines the depth of a position, in a body, at which a particle beam can arrive, and there exists a phenomenon referred to as the “Bragg peak” phenomenon in which the particle beam stops at nearly the same position as the foregoing position after emitting its energy rapidly. Therefore, by utilizing this phenomenon and appropriately adjusting the energy of a particle beam, it is made possible to kill and wound only tumor cells while suppressing the effect against normal cells existing between the surface of a body and a tumor volume. As a result, because a particle beam can intensively be irradiated onto a tumor volume, the particle-beam irradiation method is expected as a therapy method that does not impose a great deal of physical burden on a patient and less strenuous on the elderly.
On the other hand, particle-beam irradiation requires a large beam source such as an accelerator; therefore, because, unlike an X-ray source, the beam source per se cannot readily be moved, various proposals have been made in order to perform irradiation onto a diseased site at an appropriate angle. For example, in the case where a tumor in a brain or in an eyeball is treated, it is common that the treatment is performed in a horizontal irradiation chamber where a patient is seated in a chair-type patient holding apparatus, and a charged particle beam is horizontally irradiated. There has been proposed a chair-type patient holding apparatus, for radiation therapy, in which designing is performed in such a way that a charged particle beam is irradiated onto a target irradiation position referred to as an isocenter, and positioning of a patient is performed in such a way that the chair-type patient holding apparatus is moved while an X-ray radiographic image is viewed (for example, refer to Patent Document 1). There has been proposed a chair-type patient holding apparatus in which there is provided an adjustment apparatus capable of moving a patient to the center of rotation (for example, refer to Patent Document 2).
In the case of particle-beam irradiation, even though, as described above, a Bragg peak occurs, a particle beam irradiated from the outside of a body affects the surface of the body as well, to some extent. Thus, in a particle beam therapy system, in order to avoid irradiation onto an important normal tissue, it is required that the irradiation angle can appropriately be set in accordance with a diseased site. In addition, there has been proposed a multi-port irradiation method in which irradiation onto a tumor volume is performed from a plurality of directions; its effect of reducing irradiation onto a normal tissue is known.
However, in the case of a horizontal-irradiation method utilizing a conventional chair-type patient holding apparatus, it is required to change the posture of the whole chair-type patient holding apparatus with a patient supported thereon, when irradiation angle is changed; therefore, there has been a problem that a burden is imposed on the patient. For example, when the chair-type patient holding apparatus is slanted forward or laterally, a great deal of burden is imposed, especially on an aged patient.
Accordingly, there is well known a method in which, in order to realize a high-flexibility irradiation angle, not a fixed-port-irradiation type but a rotating-irradiation-type particle beam therapy system referred to as a rotating gantry is utilized in combination with a bed-type patient holding apparatus. However, this method requires a large system; thus, there has been problems that a great deal of initial cost (introduction cost) and running cost is required, that installation thereof needs a large space, and the like. A particle beam therapy system equipped with a rotating gantry is commonly utilized in proton-beam therapy; however, in the case where a heavy ion such as a carbon ion is utilized as a charged particle, the curvature radius at a time when a beam orbit is bent is large and hence a large electromagnet is required to be rotated; thus, systemization becomes further difficult. Even though multi-port irradiation can be performed by use of a rotating gantry, the irradiation apparatus is driven to rotate along with the rotating gantry when the irradiation angle is changed; therefore, there has been a problem that the irradiation angle cannot be changed unless an engineer enters the irradiation chamber and confirms that the irradiation apparatus and a patient do not collide with each other.
Therefore, there has been proposed a beam irradiation apparatus where, in order to realize flexibility in the irradiation angle without utilizing a rotating gantry, the irradiation nozzle and the scanning electromagnet are moved for each of a plurality of beam orbits defined by a deflection electromagnet (for example, refer to Patent Documents 3 and 4).
Here, in the irradiation system of a particle beam therapy system, there are required roughly two functions below. One function is to irradiating a charged particle beam onto a desired position at a desired angle, and the other function is to form an irradiation shape for selectively performing irradiation onto an irradiation subject such as a tumor. Particle beam therapy systems are divided roughly into two types, depending on the method for realizing the function of forming an irradiation shape. One of them is referred to as a broad-beam irradiation type in which irradiation onto an irradiation region is performed at once by use of an irradiation nozzle configured with a wobbler magnet, a scatterer, a range modulator, a patient collimator, a patient bolus, and the like; the other one is referred to as a scanning irradiation type in which irradiation onto an irradiation region is performed stepwise by scanning small irradiation regions with a scanning electromagnet or the like. In either types, a particle beam heads for an irradiation subject from an irradiation nozzle or a scanning electromagnet in such a way as to spread in a divergence direction; therefore, when the distance between the irradiation subject and the irradiation nozzle or the scanning electromagnet is short, the divergence angle becomes large. As a result, even in the case where irradiation onto the same irradiation subject is performed, the area of a body surface through which a charged particle beam passes becomes small in comparison with a case where the divergence angle is small; thus, the irradiation density on the body surface becomes large, whereby damage to the body surface, which is a normal tissue, increases.